1. Field of the Invention
The present invention relates to a particle therapy system for treating diseases, such as cancer and tumor, by using a charged-particle beam.
2. Description of the Related Art
In a therapy system for treating diseases, such as cancer and tumor, by irradiating protons or heavy ions, as charged particles, to a diseased area of a patient, a charged-particle beam accelerated by an accelerator, e.g., a synchrotron, is incident upon a transport means provided in a rotating (or stationary) irradiation facility, and is then introduced to an irradiation field forming apparatus. After shaping an irradiation field in match with the shape of the diseased area by the irradiation field forming apparatus, the charged-particle beam is irradiated to the diseased area of the patient lying on a patient bed that is installed below the irradiation field forming apparatus.
One example of such a therapy system is disclosed in, e.g., Japanese Unexamined Patent Application Publication No. 2001-210498. In this prior-art system, bending magnets for bending a beam direction and quadrupole magnets for adjusting a beam size are included in transport means that are provided in a high energy beam transport (HEBT) system from a synchrotron to a rotating irradiation facility and in the rotating irradiation facility. An irradiation unit serving as the irradiation field forming apparatus is provided downstream of the transport means.
As methods for forming an irradiation field by the irradiation field forming apparatus, there are conventionally known a method of enlarging a beam by using scatterers, and a beam scanning method of scanning a beam and making an amount of irradiated beam uniform through superimposition of the scanned beams.
With the method using scatterers, the beam is enlarged by employing, for example, a first scatterer made of one kind of metal and a second scatterer made of two kinds of metals having different densities. Then, beam intensity distributions resulting from those two scatterers are superimposed with each other to realize a uniform beam intensity distribution. To that end, the beam must be passed in a state in which a beam center (beam axis) coincides with a center axis of each scatterer (design orbit of the irradiation field forming apparatus), so that the beam intensity distribution resulting from each scatterer becomes symmetric about the axis.
On the other hand, with the beam scanning method, the beam is introduced to propagate in the z-direction, and varying currents are supplied to an x-direction scanning magnet and a y-direction scanning magnet to change magnetic fields generated by those magnets over time so that the beam is scanned in the x-direction and the y-direction. For example, by setting the number of scans in the x-direction per unit time to a relatively large value and the number of scans in the y-direction per unit time to a relatively small value, the irradiation field having a desired form can be formed. In this method, if the beam enters the scanning magnets in a state in which the beam axis is shifted from the design orbit, the irradiation zone is deviated from the diseased area and a uniform amount of the irradiated beam is not realized through superimposition of the scanned beams. For that reason, the beam must be introduced so as to pass predetermined positions (=design orbit of the irradiation field forming apparatus) of the two scanning magnets.
As described above, when introducing the beam from the transport means to the irradiation field forming apparatus, the beam axis is required to coincide with the design orbit of the irradiation field forming apparatus irrespective of which one of the irradiation field forming methods is employed. To that end, various components of the transport means are generally designed and arranged so that, when transporting the beam to the irradiation field forming apparatus, the beam axis is finally coincident with the design orbit of the irradiation field forming apparatus.
In practice, however, it is unavoidable that the direction of the beam axis is slightly deviated because of shape or dimension tolerances of the components of the transport means and layout or assembly errors (referred to also as “alignment errors” hereinafter). In the above-described prior-art therapy system, therefore, steering magnets are provided in a low energy beam transport (LEBT) system and the high energy beam transport (HEBT) system. Stated otherwise, though not clearly disclosed in the above-described prior art, it is usual that, assuming one direction (e.g., bending direction by the bending magnets) to be the x-direction and a direction perpendicular to the one direction (e.g., direction perpendicular to the bending direction by the bending magnets) to be the y-direction, two steering magnets for the x-direction are employed to adjust the displacement and the gradient of the beam in a plane containing the x-axis, and two steering magnets for the y-direction are also employed to adjust the displacement and the gradient of the beam in a plane containing the y-axis. With those steering magnets, the beam axis is made coincident with the design orbit of the irradiation field forming apparatus.
More specifically, the orbit is corrected through steps of, for example, installing two x-direction and y-direction monitors in the high energy beam transport (HEBT) system to detect respective displacements and gradients of the beam, and exciting the two steering magnets for each of the x-direction and the y-direction. When carrying out the operation of correcting the beam orbit, because it is unknown how large the alignment errors are, an operator has been usually required to perform adjustment on a trial-and-error basis, i.e., to manually increase or decrease bending amounts (referred to also as “kick amounts” hereinafter) of the x-direction and y-direction steering magnets as appropriate and to perform manual adjustment to coincide the beam position with the design orbit while looking at a tendency of resulting changes in the beam displacement and gradient.
Thus, in the conventional therapy system, because the beam orbit has been corrected on a trial-and-error basis while manually changing the kick amount of each steering magnet, a lot of labor and time have been required to carry out the operation of correcting the beam orbit.
Particularly, in the so-called rotating irradiation facility, as employed in the above-described prior art, wherein a rotating irradiator including the transport means and the irradiation field forming apparatus is rotatably installed about an axis of rotation so that the beam can be irradiated from a proper angular position in match with the position and condition of the diseased area, the amounts of flexures, deformations, etc. of various components caused by their own weights change depending on the rotational angle of the irradiator, and the alignment errors also change depending on the rotational angle. Hence, the operation of correcting the beam orbit must be repeated on a trial-and-error basis whenever the rotational angle of the rotating irradiator (rotating irradiation facility) is changed, thus resulting in a very troublesome operation.